1. Field of Invention
The invention relates generally to medical devices for performing mapping and ablation procedures. More particularly, the invention relates to a system for mapping and/or ablating cardiac walls.
2. Discussion of Related Art
The human heart is a very complex organ, which relies on both muscle contraction and electrical impulses to function properly. The electrical impulses travel through the heart walls, first through the atria and then the ventricles, causing the corresponding muscle tissue in the atria and ventricles to contract. Thus, the atria contract first, followed by the ventricles. This order is essential for proper functioning of the heart.
Over time, the electrical impulses traveling through the heart can begin to travel in improper directions, thereby causing the heart chambers to contract at improper times. Such a condition is generally termed a cardiac arrhythmia, and can take many different forms. When the chambers contract at improper times, the amount of blood pumped by the heart decreases, which can result in premature death of the person.
Techniques have been developed which are used to locate cardiac regions responsible for the cardiac arrhythmia, and also to disable the short-circuit function of these areas. According to these techniques, electrical energy is applied to a portion of the heart tissue to ablate that tissue and produce scars which interrupt the reentrant conduction pathways or terminate the focal initiation. The regions to be ablated are usually first determined by endocardial mapping techniques. Mapping may be active or passive. Active mapping, sometimes called “pace mapping,” typically involves percutaneously introducing a catheter having one or more electrodes into the patient, passing the catheter through a blood vessel (e.g. the femoral vein or artery) and into an endocardial site (e.g., the atrium or ventricle of the heart), and deliberately inducing an arrhythmia so that a continuous, simultaneous recording can be made with a multichannel recorder at each of several different endocardial positions. Passive mapping techniques typically involve sensing electrical signals from the electrodes on the catheter.
When an arrythromogenic focus or inappropriate circuit is located, as indicated in the electrocardiogram recording, it is marked by various imaging or localization means so that cardiac arrhythmias emanating from that region can be blocked by ablating tissue. An ablation catheter with one or more electrodes can then transmit electrical energy to the tissue adjacent the electrode to create a lesion in the tissue. One or more suitably positioned lesions will typically create a region of necrotic tissue which serves to disable the propagation of the errant impulse caused by the arrythromogenic focus. Ablation is carried out by applying energy to the catheter electrodes. The ablation energy can be, for example, RF, DC, ultrasound, microwave, or laser radiation.
Atrial fibrillation together with atrial flutter are the most common sustained arrhythmias found in clinical practice. Current understanding is that atrial fibrillation is frequently initiated by a focal trigger from the orifice of or within one of the pulmonary veins. Though mapping and ablation of these triggers appears to be curative in patients with paroxysmal atrial fibrillation, there are a number of limitations to ablating focal triggers via mapping and ablating the earliest site of activation with a “point” radiofrequency lesion. One way to circumvent these limitations is to determine precisely the point of earliest activation. Once the point of earliest activation is identified, a lesion can be generated to electrically isolate the trigger with a lesion; firing from within those veins would then be eliminated or unable to reach the body of the atrium, and thus could not trigger atrial fibrillation.
Another method to treat focal arrhythmias is to create a continuous, annular lesion around the ostia (i.e., the openings) of either the veins or the arteries leading to or from the atria, thus “corralling” the signals emanating from any points distal to the annular lesion. Conventional techniques include applying multiple point sources around the ostia in an effort to create such a continuous lesion. Such a technique is relatively involved, and requires significant skill and attention from the clinician performing the procedures.
Another source of arrhythmias may be from reentrant circuits in the myocardium itself. Such circuits may not necessarily be associated with vessel ostia, but may be interrupted by means of ablating tissue either within the circuit or circumscribing the region of the circuit. It should be noted that a complete “fence” around a circuit or tissue region is not always required in order to block the propagation of the arrhythmia; in many cases simply increasing the propagation path length for a signal may be sufficient. Conventional means for establishing such lesion “fences” include a multiplicity of point-by-point lesions, dragging a single electrode across tissue while delivering energy, or creating an enormous lesion intended to inactivate a substantive volume of myocardial tissue.
U.S. Pat. No. 6,315,778 B1, entitled “Apparatus For Creating A Continuous Annular Lesion,” which is herein incorporated by reference, discloses a medical device which is capable of ablating a ring of tissue around the ostia of either veins or arteries leading to or from the atria. The medical device includes a protrusion that inserts into an ostium, thereby allowing electrodes to contact tissue near the ostium.
In some instances, it is desirable to perform mapping and/or ablation procedures on a cardiac wall (or other tissue) that is not located near an ostium. In such a scenario, the lack of a protrusion may help to allow electrodes of a device contact the cardiac wall or other tissue. In other cases, mapping and/or ablation may be desired at several locations around an ostium and it would be helpful to be able to position electrodes without concern for a protrusion that may hinder contact between electrodes and the cardiac wall.
Another type of arrhythmia is Ventricular tachycardia. Ventricular tachycardia (VT) usually arises in diseased myocardium. However, VT can occur in the absence of structural heart disease, or at least in hearts in which current diagnostic techniques fail to identify any anatomic or functional abnormalities. These arrhythmias have been termed “idiopathic VTs”. The mechanisms underlying idiopathic VT are varied and include reentry and triggered activity due to delayed after depolarizations.
Idiopathic VTs that arise from the right or left ventricular outflow tract (RVOT VT and LVOT VT) have been reported. Thus RVOT VT and LVOT VT patients could be treated with RF ablation. However, the success rate of ablation therapy for treatment of VT is affected by many factors, such as the inability to induce tachycardia to permit mapping, and the presence of deep, often septal sites of origin that are resistant to RF ablation with conventional ablation catheters, usually a 4-mm ablation catheter. Treating VT in the area of the outflow track with ablation therapy has been difficult.